The invention relates to a process for the manufacture of a milk product having a reduced spores and bacteria content, especially a process for treating milk to produce standardized milk for consumption or cheese making, or skim milk powder or whole milk powder as well as a plant for carrying out the process, and also a process for the manufacture of whey or pre-concentrated whey having a reduced spores and bacteria content, and to a plant for performing the process
In the production of standardized milk for consumption or cheese making or milk powder the starting material, viz. the milk, is conventionally separated into a cream fraction and a skim milk fraction before the subsequent processing. The cream fraction which has a high content of spores and bacteria (microorganisms) is subjected to a heat treatment suitable to obtain a maximum killing of these microorganisms (sterilization). In parallel to the treatment of the cream fraction, the skim milk fraction which also contains spores and bacteria (microorganisms) and fat is subjected to a microfiltration resulting in a separation into a retentate carrying an increased content of microorganisms and fat and a permeate consisting of skim milk having reduced microorganism and fat content. The sterilized cream fraction and the permeate consisting of skim milk of reduced microorganism and fat content is brought together and mixed in a defined ratio to produce the desired milk products.
It is an advantage of the prior art processing method that only a minor portion of the milk product, viz. the cream fraction, has to be sterilized to obtain a standardized milk for human consumption or cheese making having a reduced content of microorganisms. Thus, the permeate consisting of skim milk having a reduced content of microorganisms and fat needs not to be heated when the content of such micro biological contaminations are sufficiently low. By omitting this heating, a more or less pronounced conversion of the proteins in the permeate due to denaturation is avoided and also the complete or partial destruction of enzymes in the permeate is prevented.
It is well-known that such changes of the milk inter alia influence the curdling ability of the milk when used in cheese making. This may for instance result in liberation of moisture during the subsequent curing of the cheese, or if the milk has been subjected to intensive heating it may become completely unsuitable for the manufacture of cheese.
The possibility of omitting a heat treatment or at least to use only a lenient heat treatment of the permeate due to the low content therein of the microorganisms influencing the shelf-life of the milk product (bacteria, yeasts, moulds, virus, and bacteriophages) is advantageous in connection with the production of standardized milk for human consumption. Hereby the desired organoleptic and nutritional characteristics of the milk are maintained at least in this component of the mixture.
Besides, the reduction of the microfiltration to the skim milk fraction alone involves a considerably increased capacity of the microfilter.
It is an object of the invention to provide a process according to the preamble of claim 1. A process of this type is known from EP 0 194 286 B1 or U.S. Pat. No. 4,876,100. However, this process is restricted as far as the recovering of the cream fraction and the skim milk fraction is concerned to the use of a preliminary centrifugal separation of the milk in spatial vicinity to the subsequent treatment. By said process the microfiltration of the skim milk is especially important. The skim milk is during the microfiltration recycled in a loop including a microfilter arranged so that the skim milk flows parallel to the surface of the filter membrane (crossflow filtration). The microfilter is provided with pipes and connected in a conventional manner, i.e. the microfiltration unit consisting of the microfilter and the pipe connections belonging thereto is supplied with the skim milk to be filtered through a feed conduit, and the permeate passing through the microfiltration membrane as well as the concentrate (the retentate) which builds up in the flow loop is recovered from the microfiltration unit through conduits connected thereto. Subsequently, the retentate is sterilized. In one embodiment of the plant, a recycling loop is connected to the permeate exit from the microfilter and a pump is used for recycling permeate from the microfilter to the inlet of the circulation loop. It is not described in which ratio any permeate is recycled to the circulation loop of the one-step microfiltration unit, and for which purpose such a recycling is performed and which result is obtained thereby.
A further process of the type in question is known from EP 0 697 816 B1. Also in this process, the cream fraction and the skim milk fraction are recovered by a centrifugal separation of the milk in spatial vicinity to the subsequent treatment. By this process, the concentrate containing spores and bacteria (microorganisms) which leaves the microfiltration unit as retentate is recycled to the centrifugation and mixed with the milk fed to the centrifugal separator.
The process of EP 0 697 816 B1 can only be used when the cream and skim milk fractions are recovered from a milk centrifugal separator connected prior to the plant for their further treatment and spatially near said plant. Thus, it cannot be used when for instance the two fractions are provided from feed vessels connected upstream of the equipment for further treatment spatially near thereto.
WO 91/09667 discloses a process for performing tangential microfiltration, reverse osmosis or ultra-filtration using a two-stage plant. No information is given on the envisaged field of application. In this process, a retentate formed in a first step is used as feed for a second step. It is not specified how the permeates from the two steps are used or handled. A process of this type, and in which the two permeates are pooled, is depicted in FIG. 3 on the drawing attached hereto.
The amount of germs in the final products of prior art processes using microfiltration within the dairy industry is unsatisfactory due to the skim milk component available for mixing with the cream fraction. For the above-mentioned reasons (complete omission of a heat treatment or only a lenient heat treatment) it is desired and attempted to reduce the content of microorganisms in the available skim milk as far as possible since these microorganisms have a decisive influence on the shelf-life of the milk product.
From WO 00/74495 and U.S. Pat. No. 5,685,990 it is known to perform filtration, incl. microfiltration, using two or more steps. The permeate from one step is conducted to one or more steps, meaning that it passes through at least two membranes. Thereby an efficient and safe purification is obtained but at the cost of capacity/required filter area. Another drawback is the repeated recircling of the fraction having the highest bacteria content.
The drawbacks of the above briefly described known processes for producing standardized consumer or cheese milk also exists when skim milk powder or whole milk powder is the final milk product. An object of the present invention is to provide a process of the type dealt with by which it is possible to produce the desired milk product via a skim milk having a reduced content of microorganisms (germs and bacteria) compared to the microoganism content in prior art processes. Moreover, the invention makes it possible to produce whey or pre-concentrated whey having a reduced spores and bacteria content by using characteristic features thereof.
The object of the invention is achieved by a process having the characteristics defined in claim 1. Preferred embodiments of the proposed process are subject of the sub-claims. Independent claim 1 characterizes a process for producing a whey product or having a reduced spores and bacteria content.
According to the invention, the process is carried out using a two-steps microfiltration. A first permeate formed in the first step of the microfiltration, which permeate consists of the skim milk having a reduced content of microorganisms and fat, is mixed with the sterilized cream fraction. The second step of the microfiltration is fed with a retentate from the first step, and a second permeate formed in the second step is completely or partially recycled to the skim milk fraction at the inlet into the first step.
By the process according to the invention, it is possible to reduce the content of microorganisms and fat in the resulting permeate significantly compared to the corresponding content of microorganisms and fat in the permeate from prior art processes. Relevant comparison data can be found in the below example calculations (Tables 1 and 2).
A further advantage of the process of the invention is that the reduced content of microorganisms in the permeate which has an essential influence on the shelf-life of the milk product, can be obtained without extra investments in the equipment. By an embodiment of the proposed process, the content of microorganisms in the permeate can be influenced within some limits when a part of the second permeate is added directly to the first permeate.
The proposed process is generally applicable under operational conditions in which the cream and skim milk fractions are recovered from a centrifugal separation of the milk performed spatially near the further treatment. In this case, these two fractions can be supplied to the further treatment directly from the upstream centrifugal separation, or the two fractions may also be stored following the centrifugal separation and only conducted to further treatment at a later time and/or at another location.
A further advantage of the proposed process is that it can also be used in such operational circumstances where the two fractions are provided from a stock spatially near and upstream of their further treatment, i.e. without direct or indirect spatial relation to the centrifugal separation of the milk.
The essential feature of the proposed plant for performing the process of the invention is a two-steps microfiltration unit. The two steps are performed in a manner known per se as known from the relevant prior art, as far as the construction of the microfiltration device is concerned, and is sufficiently described in the prior art (cf. e.g. Socixc3xa9txc3xa9 de Ceramique Techniques (SCT), MEMBRALOX(copyright) USERS"" MANUAL). In a first embodiment, the two-steps microfiltration unit is connected directly to a centrifugal separator. Furthermore, a stock vessel for the relevant fractions can be inserted in the feed conduit for the cream fraction and in the feed conduit for the skim milk fraction from the centrifugal separator. Moreover, the two fractions can, according to a further embodiment, each be taken from a stock vessel independent on the presence of a centrifugal separation unit.
The first microfiltration section and the second microfiltration section each have at least one microfiltration module. Each microfiltration module has at least one microfiltration element sealed in a housing having connections for the liquid to be filtered as well as for the permeate and the retentate. The microfiltration element is preferably made of a porous carrier material permeable for the permeate and it has at least one duct through which the liquid to be filtered is flowing, the wall of which duct forms the carrier surface for the microfiltration membrane.
The operational period for such microfiltration elements and the so-called flux through the microfiltration membrane of such elements is favourably influenced or increased when one or more of the features of the microfiltration element construction, such as the gradients of the thickness of the active membrane layer, the porosity of the membrane layer and/or support, when seen over the length subjected to the liquid flow compensates for the change of the pressure difference between the side for the liquid to be filtered and the permeate side, as is the case in a preferred embodiment of the invention.
The microfiltration sections of the plant according to the invention are equipped with the same or different microfiltration membranes having a pore size of 0.05-5.0 xcexcm. For the manufacture of standardized milk for human consumption or for cheese making it is preferred to use a pore size of 0.8-1.4 xcexcm. For other applications smaller pore sizes, down to 0.05 xcexcm can be used.
The operational period and the so-called flux for the microfiltration membrane are favourably influenced and becomes optimal when the crossflow velocity tangentially over the microfiltration membrane in the circulation loop of the first and/or the second microfiltration unit is in the range of 4 less than v less than 10 m/s, preferably v≈6 m/s.
Basically, all available membrane materials suitable for the desired milk or whey product can be used. Especially satisfactory results are obtained in embodiments using ceramic membrane materials having high chemical resistance. This is especially important during the cleaning procedures (CIP-cleaning) necessarily carried out at fixed intervals.
The inventive features of the proposed process comprise a further important application, viz. the decrease of the spores and bacteria content in whey or pre-concentrated whey without subjecting these wheys to a thermal treatment. This is important since the constituents of the whey, such as whey proteins and lactose, are of nutritional value and should be converted as little as possible. Besides, structural developments in the cheese industries have resulted in the production of larger amounts of whey than can be used directly as animal feed. Due to the content of organic components, such as proteins and lactose, the disposal as waste water is not allowed.
By this application of the present process, the spores and bacteria containing whey is subjected to a microfiltration similar to the above described treatment of the skim milk fraction obtained from the milk. The whey is separated in a manner known per se in a retentate having an increased content of spores and bacteria and in a permeate having a reduced content of such microorganisms. The microfiltration of the microorganisms containing whey is performed in two steps, whereby the first step generates a first permeate consisting of whey having a reduced content of microorganisms. The subsequent second step is supplied with a retentate from the first step, and a second permeate formed in the second step is recycled completely or partially to the inlet of the first step.
According to an embodiment of the proposed process, the content of microorganisms in the permeate can also in this application be changed within some limits if a portion of the second permeate is recycled directly to the first permeate.
The plant for performing the process to produce whey having a reduced content of spores and bacteria corresponds as far as the here relevant microfiltration unit is concerned, to the plant used for treatment of the skim milk fraction obtained from the milk. Also the variations of the plant described for this purpose are relevant for the plant used for whey treatment.
It can be concluded that the embodiments of the invention in the various applications thereof (milk or whey products) besides the above described advantages, such as reduction of the spores and bacteria content in milk or whey products, always involves reduction of operating costs and energy consumption relative to the prior art processes and plants.